Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase.
Methods for the chemical synthesis of butanols are known. For example, 1-butanol may be produced using the Oxo process, the Reppe process, or the hydrogenation of crotonaldehyde (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719). 2-Butanol may be produced using n-butene hydration (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719). Additionally, isobutanol may be produced using Oxo synthesis, catalytic hydrogenation of carbon monoxide (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719) or Guerbet condensation of methanol with n-propanol (Carlini et al., J. Molec. Catal. A: Chem. 220:215-220 (2004)). These processes use starting materials derived from petrochemicals, are generally expensive, and are not environmentally friendly.
Methods of producing butanol by fermentation are also known, where the most popular process produces a mixture of acetone, 1-butanol and ethanol and is referred to as the ABE processes (Blaschek et al., U.S. Pat. No. 6,358,717). Acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum is one of the oldest known industrial fermentations, and the pathways and genes responsible for the production of these solvents have been reported (Girbal et al., Trends in Biotechnology 16:11-16 (1998)). Additionally, recombinant microbial production hosts expressing a 1-butanol biosynthetic pathway (Donaldson et al., copending and commonly owned U.S. Patent Application Publication No. US20080182308A), a 2-butanol biosynthetic pathway (Donaldson et al., copending and commonly owned U.S. Patent Application Publication Nos. US20070259410A1 and US20070292927A1 US 20070292927A1), and an isobutanol biosynthetic pathway (Maggio-Hall et al., copending and commonly owned U.S. Patent Publication No. US 20070092957) have been described. However, biological production of butanols is believed to be limited by butanol toxicity to the host microorganism used in the fermentation.
In addition, 2-butanone is a valuable compound that can be produced by fermentation using microorganisms. 2-Butanone, also referred to as methyl ethyl ketone (MEK), is a widely used solvent and is the most important commercially produced ketone, after acetone. It is used as a solvent for paints, resins, and adhesives, as well as a selective extractant and activator of oxidative reactions. In addition, it has been shown that substantially pure 2-butanone can be converted to 2-butanol by reacting with hydrogen in the presence of a catalyst (Nystrom, R. F. and Brown, W. G. (J. Am. Chem. Soc. (1947) 69:1198). 2-butanone can be made by omitting the last step of the 2-butanol biosynthetic pathway (Donaldson et al., copending and commonly owned U.S. Patent Application Publication Nos. US20070259410A1 and US20070292927A1). US 20070292927A). Production of 2-butanone would be enhanced by using microbial host strains with improved tolerance as fermentation biocatalysts.
Strains of Clostridium that are tolerant to 1-butanol have been isolated by chemical mutagenesis (Jain et al. U.S. Pat. No. 5,192,673; and Blaschek et al. U.S. Pat. No. 6,358,717), overexpression of certain classes of genes such as those that express stress response proteins (Papoutsakis et al. U.S. Pat. No. 6,960,465; and Tomas et al., Appl. Environ. Microbiol. 69(8):4951-4965 (2003)), and by serial enrichment (Quratulain et al., Folia Microbiologica (Prague) 40(5):467-471 (1995); and Soucaille et al., Current Microbiology 14(5):295-299 (1987)). Desmond et al. (Appl. Environ. Microbiol. 70(10):5929-5936 (2004)) report that overexpression of GroESL, two stress responsive proteins, in Lactococcus lactis and Lactobacillus paracasei produced strains that were able to grow in the presence of 0.5% volume/volume (v/v) [0.4% weight/volume (w/v)] 1-butanol. Additionally, the isolation of 1-butanol tolerant strains from estuary sediment (Sardessai et al., Current Science 82(6):622-623 (2002)) and from activated sludge (Bieszkiewicz et al., Acta Microbiologica Polonica 36(3):259-265 (1987)) has been described. Additionally some Lactobacillus sp are known to be tolerant to ethanol (see for example, Couto, Pina and Hogg Biotechnology. Letter 19: 487-490) Ingram and Burke (1984) Adv. Micribial. Physiol 25: 253-300. However, for most bacteria described in the art, growth is highly inhibited at low concentrations of 1-butanol. Moreover butanol is much more toxic than ethanol and mechanisms that affect the ethanol tolerance of E. coli have not been found to affect the butanol response.
There is a need, therefore, for butanol or 2-butanone producing bacterial host strains that are more tolerant to these chemicals, as well as methods of producing butanols or 2-butanone using bacterial host strains that are more tolerant to these chemicals.